The present invention is directed to a process for preparing a diene elastomer having a reduced amount of vinyl linkages by anionic polymerization of one or more monomers comprising at least one conjugated diene monomer.
A conjugated diene monomer such as butadiene may be incorporated into an anionically produced polymer via cis-1,4, trans-1,4 or -1,2(vinyl) linkages.
Anionic polymerization reactions are conventionally initiated by an alkyllithium, in the presence of a hydrocarbon solvent and one or more conjugated diene monomers. Elastomers produced by these reactions have an average amount of vinyl linkages which is generally between 8% and 15%. In order to obtain elastomers having a given glass transition temperature (Tg), a polar agent is added to the polymerization medium, which has the effect of significantly increasing the average amount of vinyl linkages in the polymer. This amount may reach 90%, for example.
Attempts have been made to reduce the increased amount of vinyl linkages in the polymer during polymerization, when polymerization is performed in the presence of a polar agent, by acting on the polymerization medium.
One known method involves increasing the polymerization temperature and using a polar agent formed of a Lewis base, optionally in the presence of a Lewis acid. The amount of vinyl linkages in the copolymer obtained using this method can be reduced to about 20% to 50%. Reference may be made to U.S. Pat. No. 3,830,880 and JP-A-56/149413 for detailed descriptions of such a method. However, a major disadvantage of this method lies in the chain transfers which the implementation thereof generally causes.
Another known method for reducing the amount of vinyl linkages involves eliminating the polar agent from the polymerization medium under reduced pressure, then adding an additional quantity of monomer to the polymerization medium thus obtained. Reference may be made to U.S. Pat. No. 3,140,278 for the description of this method. However, this method is only applicable to copolymerization operations carried out in the presence of a volatile polar agent. Moreover, the reduction in the amount of vinyl linkages in the resulting polymers is not significant. This method is also difficult to implement on an industrial scale.
The object of the present invention is to provide an anionic polymerization process for the preparation of a diene elastomer which overcomes the aforementioned disadvantages.
It has been discovered that the addition, to a polymerization medium comprising:
a hydrocarbon solvent,
one or more conjugated diene monomers,
a polar agent comprising one or more heteroatoms, and
a monolithiated or polylithiated organolithiated initiator,
of an organometallic complex of the formula LiAl(R)3(Rxe2x80x2) (in which Li is a lithium atom, Al an aluminum atom, and R and Rxe2x80x2 are each an alkyl, cycloalkyl or aryl group), makes it possible to oppose the effect of the polar agent by significantly reducing the average amount of vinyl linkages in the elastomer obtained, independent of the nature of the polar agent and the initiator which are used.
The addition of the organometallic complex to the polymerization medium has no significant effect on the molecular weights, or the distributions, of the elastomers produced.
Moreover, the addition of an additional quantity of polar agent subsequent to the introduction of the organometallic complex into the reaction medium has the effect of again increasing the average amount of vinyl linkages in the diene elastomer. This provides the ability during polymerization to adjust the amount of vinyl linkages in the polymer product to a desired average amount.
The present invention provides a process for preparing a diene elastomer having a reduced amount of vinyl linkages. The process involves anionically polymerizing one or more monomers comprising at least one conjugated diene monomer in a polymerization medium comprising a hydrocarbon solvent, a polar agent having one or more heteroatoms and a monolithiated or polylithiated organolithiated initiator, wherein an organometallic complex of the formula LiAl(R)3(Rxe2x80x2), in which Li is a lithium atom, Al is an aluminum atom, and R and Rxe2x80x2 are each an alkyl, cycloalkyl or aryl group, is added to the polymerization medium to produce the diene elastomer having a reduced amount of vinyl linkages.
Hydrocarbon solvents usable in the process according to the invention include, inter alia, toluene, benzene, xylene, cyclohexane, methylcyclohexane, heptane, n-hexane, cyclopentane, or mixtures of these solvents.
In accordance with the present invention, suitable conjugated diene monomers for use in the process include 1,3-butadiene, 2-methyl-1,3-butadiene, 2,3-di(C1 to C5 alkyl)-1,3-butadienes, for example, 2,3-dimethyl-1,3-butadiene, 2,3-diethyl-1,3-butadiene, 2-methyl-3-ethyl-1,3-butadiene and 2-methyl-3-isopropyl-1,3-butadiene, an aryl-1,3-butadiene, 1,3-pentadiene and 2,4-hexadiene.
Suitable vinyl-aromatic compounds include styrene, ortho-, meta- and para-methylstyrene, the commercial mixture xe2x80x9cvinyltoluenexe2x80x9d, para-tert.-butylstyrene and divinylbenzene. Styrene is preferred.
The diene elastomer prepared by the process of the invention may be any homopolymer or copolymer having diene blocks which is obtained by polymerization:
of a conjugated diene monomer having from 4 to 12 carbon atoms or of several of these conjugated diene monomers together, or alternatively
of one or more of these conjugated diene monomers with one or more vinyl-aromatic compounds, each having from 8 to 20 carbon atoms.
Particularly preferably, butadiene and/or isoprene is/are used as conjugated diene monomers and styrene as vinyl-aromatic monomer in the process according to the invention to produce copolymers having diene blocks, each block resulting from the monomers and comprising, for example, units:
of styrene and butadiene,
of styrene and isoprene,
of butadiene and isoprene or, alternatively
of butadiene, styrene and isoprene.
Advantageously, the diene blocks of the elastomer which is obtained by the process according to the invention have amounts of vinyl linkages which significantly differ from each other.
The anionic polymerization process of the invention allows for production of a diene elastomer comprising a plurality of alternating diene blocks that which have amounts of vinyl linkages that are very different from each other (for example, an elastomer of type Axe2x80x94Bxe2x80x94Axe2x80x94B, in which A and B are two diene blocks containing high and reduced amounts of vinyl linkages, respectively), by alternately adding the polar agent and the organometallic complex to the polymerization medium at given conversion rates.
Additionally, the use of the organometallic complex according to the invention makes it possible to prepare elastomers having a reduced average amount of vinyl linkages, following the preparation of elastomers having a high average amount of vinyl linkages, without having to extract the polar agent providing this high amount from the solvent. This is because of the xe2x80x9canti-polarxe2x80x9d effect for said amount of the organometallic complex according to the invention.
In a further aspect of the invention, the organometallic complex may be added at any stage of polymerization, including prior to introduction of the initiator to the reaction medium.
The organometallic complex may be prepared by reacting an alkyllithium with a trialkylaluminium derivative in a hydrocarbon solvent at ambient temperature.
In a preferred embodiment, the organometallic complex has the formula LiAl(R)3(Bu), in which Bu designates a butyl group.
In an example of this preferred embodiment, the organometallic complex has the formula LiAl(Oct)3(Bu), in which Oct designates an octyl group. This complex comprising an octyl group has particularly high solubility in aliphatic or alicyclic hydrocarbon solvents, such as cyclohexane.
In another example of this preferred embodiment, the organometallic complex has the formula LiAI(Et)3(Bu), in which Et designates an ethyl group.
In accordance with the invention, the molar ratio of organometallic complex/polar agent varies from 0.1 to 10. The molar ratio of polar agent/initiator also varies from 0.1 to 10.
Polar agents that may be used in the process according to the invention include polar agents comprising a single heteroatom (oxygen or nitrogen atom), also referred to as xe2x80x9cmonodentatexe2x80x9d, as well as polar agents comprising several heteroatoms, also referred to as xe2x80x9cmultidentatexe2x80x9d.
A xe2x80x9cmonodentatexe2x80x9d polar agent for use in the process according to the invention is tetrahydrofuran (THF).
xe2x80x9cMultidentatexe2x80x9d polar agents for use in the process include diethers and diamines, such as tetramethylethylenediamine, dimethoxyethane (DME), diethylcarbitol (DEC), triethyleneglycoldimethylether (also called xe2x80x9ctriglymexe2x80x9d), tetraethyleneglycoldimethylether (also called xe2x80x9ctetraglymexe2x80x9d), and tetramethylethylenediamine (TMEDA).
xe2x80x9cMultidentatexe2x80x9d polar agents are preferred according to the invention because they make it possible to obtain high average amounts of vinyl linkages, even when their concentration is reduced compared to that of the initiator.
Moreover, when a xe2x80x9cmultidentatexe2x80x9d polar agent is used, the organometallic complex according to the invention permits a significant reduction in the average amount of vinyl linkages in the elastomer, even when it is used in a quantity which is not very high relative to that of the polar agent (for example, by adding 1 molar equivalent of the complex to 1 equivalent ofxe2x80x9cmultidentatexe2x80x9d polar agent).
In regard to a styrene-butadiene copolymerization carried out in the presence of a xe2x80x9cmultidentatexe2x80x9d polar agent, the addition of an appropriate quantity of the organometallic complex to the polymerization medium provides amounts of styrene and vinyl linkages in the resulting elastomer which are comparable to those which would have been obtained in the absence of polar agent.
Organolithiated initiators that may be used in the process of the invention include monolithiated compounds, such as n-butyllithium, sec-butyllithium, tert-butyllithium, n-propyllithium, amyllithium, cyclohexyllithium or phenylethyllithium. Polylithiated or functional initiators may also be used.
According to another aspect of the invention, polymerization is carried out at a temperature of between 0xc2x0 C. and 100xc2x0 C., preferably, between 30xc2x0 C. and 80xc2x0 C.
According to the invention, the polymerization process may be carried out continuously or discontinuously. For continuous polymerization, a system preferably comprising several reactors is used, wherein the organometallic complex is injected between two reactors. During discontinuous polymerization, the organometallic complex may be injected into the polymerization medium at any stage of the conversion of the monomer(s).